1. Field of the Invention
This invention relates to a process for the modeling and control of distributed factories which have fabrication sequences. It further relates to a class of sequenced-distributed-factory (SDF) dynamic models. It also relates to factory-specific models from the SDF class and to the use of such factory specific model to generate factory schedules and to control material movement and machine loadings in the factory. The invention further relates to a computer integrated manufacturing (CIM) system that includes a factory specific model for automatic control and to the use of SDF models for the design of CIM systems.
2. Description of the Prior Art
Most manufacturing plants or factories are distributed in that they consist of heterogeneous, unconnected workstations. The virtue of this factory design is that it provides adaptability to a varying product mix. The drawback is the resulting complexity of operations, management, and quality control.
A distributed manufacturing plant is capable of fabricating a variety of products through ordered-process sequences of process steps. Each process step can be performed by at least one workstation in the factory. Distributed factories are common in the manufacture of modern electronics products. Six types of distributed factories can be involved: wafer slicing, wafer fabrication, semiconductor-component assembly, circuit-board fabrication, circuit-board assembly, electronic-product assembly.
The archetype of a distributed factory is a wafer-fabrication plant, or "wafer fab," which may manufacture products simultaneously according to over one thousand processes, averaging over one hundred steps each. An example of wafer fabrication is described in detail below.
We are now in position to illustrate the complexity of distributed factories. For example, wafer fabrication factories with over one thousand fabrication sequences are known. Such a large collection of processes is difficult to represent in a drawing like a fab graph. Such a factory, however, can be described to a computer system.
The complexity of distributed factories is further illustrated by the existence of tens of thousands of fabrication sequences in a general class of distributed factory called a "job shop." The standard approach to describing the collection of sequences in a job shop is to surrender to the complexity and describe the product paths through the factory as being random. They are clearly not random, but only recently have computers provided the practical computational power to describe highly-complex factories accurately.
The factory is a complex, data- and information-rich entity. A data structure with tens of thousands of parameters may be required merely to describe the factory. Furthermore, in operation a dynamic factory produces orders of magnitude more data describing the production flows. The sheer volume of information has made operation and control of distributed factories a major problem.